There are three major isoforms of the human membrane folate binding protein, α, β, and γ. The α and β isoforms have about 70% amino acid sequence homology, and differ dramatically in their stereospecificity for some folates. Both isoforms are expressed in fetal and adult tissue, although normal tissue generally expresses low to moderate amounts of FR-β. FR-α, however, is expressed in normal epithelial cells, and is frequently strikingly elevated in a variety of carcinomas (Ross et al. (1994) Cancer 73(9):2432-2443; Rettig et al. (1988) Proc. Natl. Acad. Sci. USA 85:3110-3114; Campbell et al. (1991) Cancer Res. 51:5329-5338; Coney et al. (1991) Cancer Res. 51:6125-6132; Weitman et al. (1992) Cancer Res. 52:3396-3401; Garin-Chesa et al. (1993) Am. J. Pathol. 142:557-567; Holm et al. (1994) APMIS 102:413-419; Franklin et al. (1994) Int. J. Cancer 8 (Suppl.):89-95; Miotti et al. (1987) Int. J. Cancer 39:297-303; and Vegglan et al. (1989) Tumori 75:510-513). FR-α is overexpressed in greater than 90% of ovarian carcinomas (Sudimack and Lee (2000) Adv. Drug Deliv. Rev. 41(2):147-62). FR-α generally attaches to the cell surface membrane via a GPI anchor. GPI anchors contain oligosaccharides and inositol phospholipids.
In 1987, Miotti et al. described three new monoclonal antibodies that recognized antigens on human ovarian carcinoma cells (Miotti et al. (1987) Int. J. Cancer 39(3):297-303). One of these was designated MOv18, which recognizes a 38 kDa protein on the surface of choriocarcinoma cells. MOv18 is a murine, IgG1, kappa antibody and mediates specific cell lysis of the ovarian carcinoma cell line, IGROV1. Alberti et al. ((1990) Biochem. Biophys. Res. Commun. 171(3):1051-1055) showed that the antigen recognized by MOv18 was a GPI-linked protein. This was subsequently identified as the human folate binding protein (Coney et al. (1991) Cancer Res. 51(22):6125-6132). Tomassetti et al. showed that MOv18 recognizes a soluble form and a GPI-anchored form of the folate binding protein in IGROV1 cells (Tomassetti et al. (1993) FEBS Lett. 317(1-2):143-146). Subsequent work combined the variable regions of the mouse MOv18 with human IgG1 (kappa) constant region to create a chimerized MOv18 antibody. The chimerized antibody mediated higher and more specific lysis of IGROV1 cells at 10-100-fold lower antibody concentrations (Coney et al. (1994) Cancer Res. 54(9):2448-2455). The 38 kDa antigen appears to be the monomeric form of FR-α.
U.S. Pat. No. 5,952,484 describes a humanized antibody that binds to a 38 kDa protein (FR-α). The antibody was named LK26. The original mouse monoclonal antibody was described by Rettig in European Patent Application No. 86104170.5 (published as EP0197435 and issued in the U.S. as U.S. Pat. No. 4,851,332).
Ovarian cancer is a major cause of death due to gynecological malignancy. Although chemotherapy is the recommended treatment and has enjoyed some success, the 5-year survival rate is still less than 40%.
A difficult problem in antibody therapy in cancer is that often the target of the antibody is expressed by normal tissues as well as cancerous tissues. Thus, the antibodies that are used to kill cancer cells also have a deleterious effect on normal cells. Finding unique targets or targets that are preferentially expressed in cancer tissues has proven difficult in many cancers. Identification of preferentially expressed targets and the ability to block the biological activity of such targets may be an effective treatment for cancer. As such, more effective antibody therapies for ovarian and other FR-α-bearing cancers that avoids or minimizes reactivity with normal tissues are needed.